As a representative standard protocol in an in-vehicle network of automobiles, a CAN (Controller Area Network) is in widespread use. With this kind of in-vehicle network, there is a possibility that an unauthorized device may be connected to an interface which is directly connected to an in-vehicle network such as an OBD 2 (On-Board-Diagnostics 2) port, and a replay attack may be carried out from such unauthorized device. Here, the term “replay attack” refers to an attack of intercepting and acquiring a message on a communication path in advance, and retransmitting the acquired message to induce unauthorized operations. Moreover, there is also a possibility that an information processing device linked with a system outside the vehicle may be infected with malware.
Normally, to deal with the foregoing threats, it would be effective to perform message authentication using a MAC (Message Authentication Code) as a falsification detection code to the message being transferred between the respective information processing devices. For instance, Japanese Laid-Open Patent Publication No. 2013-098719 (PTL 1) discloses a communication system which embeds a MAC in a message on an in-vehicle network. With the communication system described in PTL 1, each information processing device counts the number of times that a message was sent for each message ID. The transmitting-side information processing device generates a MAC from the data, number of transmissions, and message ID. The receiving-side information processing device calculates the MAC based on the data, number of transmissions and message ID in the received message, and compares the calculated MAC with the separately received MAC. When the calculated MAC and the received MAC are different, the receiving-side information processing device takes measures against replay attack and malware infection while subsequently refraining from receiving messages of that message ID.